This invention relates to magnetic head transducers and in particular to magnetic head transducers having precisely contoured recording surfaces.
In recording or reading information on a moving information storage media, such as a magnetic recording disc, relative motion between a magnetic head transducer used for reading and writing information and the storage media on which information is written on and read from is required. The relative velocity between the head and media interface may be, for example, 550 IPS for video and high density digital applications. To achieve maximum performance, an interface of intimate contact is provided between the head and media surface without destruction or excessive wear of the magnetic flux responsive coating on the surface.
As contact pressure between the head and media is increased to improve performance, both media and head wear are increased. The problem is particularly severe in video applications and compounded with the medias utilized for storing single video frames on separate tracks of a magnetic disc wherein one track at a time is continuously in contact with the head to provide a continuous stationary display of a single frame. In only five minutes of playtime revolving at 3600 RPM, for example, a track on the disc is scraped about 18,000 times by the head; the by-products are so hard and abrasive that the same materials are commonly used as a lapping compounds.
To prevent failure caused by contact between the head and disc, lubricated surfaces and/or air film separations have been used. However, any separation between the head and disc caused by such lubrication fluid or air film imposes a loss of signal and hence performance. A head/disc separation equal to one wave length could cause about 54.6 dB loss in the output of the replay head. Since it is desired to record wave lengths that approach 1.75 microns, the playback head voltage is reduced to 50% by only 0.19 micron of separation. On the other hand, as discussed above, reduction of separation to meet desired preformance would cause the interface to be destroyed within a few seconds.
Prior art solutions to the head-to-disc interface problem have generally been of two types: flying heads in conjunction with rigid hard-surface discs and heads having large surface areas buried in soft flexible "floppy" discs. Flying head discs are very expensive and require complicated and expensive recording/playback systems. A flexible or so-called "floppy" disc reduces the handling and cost problems incurred in the flying head rigid-disc system. Some record/read heads for "floppy" discs are relatively large to provide an interface comprised of a large contoured head buried in the soft flexible media. The large record/read head surface area distributes the force per unit area to reduce media wear and separation loss. As the media is moved past the head, however, air collects between the head and disc surface to form an air film. The thickness of this air film is a function of head and media surface finishes, media stiffness, head-media penetration, head size, head surface contour, viscosity of the air and disc-head relative velocity. Because of these restraints, most flexible or "floppy" disc applications are limited to slow speed, low bandwidth digital computer applications or voice recording systems.
In high performance recording systems the shape of the magnetic head transducer is important in achieving optimum system performance. Copending Patent Application Ser. No. 45,528, filed of even date with and assigned to the Assignee of the present invention, describes the advantages obtained from using a head having a one inch radius of curvature on the recording surface thereof in achieving intimate head to disc contact to optimize signal transfer between the head and disc while maintaining sufficient separation therebetween to minimize wear. Because close tolerances of the dimensions of the head are important, precise contouring of the recording surface of the head must be achieved. Prior art techniques for shaping the recording/playback surface of the head have generally involved the use of abrasive tapes of varying degrees of abrasiveness to grind and polish the head. The most abrasive tapes are used to grind the head while the less abrasive tapes are used to polish the head and further refine the contouring. One problem encountered with this method is that the amount of penetration of the tape into the various heads often varied from head to head, thereby producing heads with non-uniform contours.